Herpesviruses are large double stranded DNA viruses consisting of an icosahedral capsid surrounded by an envelope. The group has been classified as alpha, beta and gammaherpesviruses on the basis of genome structure and biological properties [Roizman, B et al (1981) Inter-virology 16, 201-217]. Avian herpes viruses include Marek's Disease Virus (MDV) (a gammaherpesvirus) which causes a lymphomatous disease of considerable economic importance in chickens [reviewed in Payne, L. N. (ed) Marek's Disease (1985), Martinus Nijhoff Publishing, Boston] and Infectious Laryngotracheitis-Virus (ILTV) (an alphaherpesvirus) which causes an acute upper respiratory tract infection in chickens resulting in mortality and loss of egg production.
A recent unexpected finding in out laboratory is that there is sufficient amino acid homology between MDV, ILTV and mammalian herpesviruses, particularly varicella zoster (VZV) and Herpes Simplex Virus (HSV) to allow identification of numerous conserved genes. These include the MDV and Herpesvirus of Turkeys (HVT) homologues of glycoproteins gB, gC and gH of HSV; the ILTV, MDV and HVT homologues of TK and ribonucleotide reductase genes and the ILTV homologue of gB and genes 34 and 35 of VZV [Buckmaster, A et al, (1988) J. gen. Virol, 69, 2033-2042.
Strains of MDV have been classified into three serotypes. Type 1 comprises pathogenic strains and their attenuated derivatives. Type 2 are a group of naturally-occurring non-pathogenic strains and type 3 is HVT. For more than a decade, vaccination with HVT has been remarkably effective in controlling Marek's disease. However, in recent years, new strains of MDV have been isolated which cause disease despite vaccination with HVT. Losses due to these `very virulent` strains have occurred in parts of the U.S.A., Europe and the Middle East. Although the degree of protection can be improved by using a mixture of HVT, type 2 MDV and attenuated derivatives of very virulent strains for vaccination, the results have been erratic.. These observations and the fact that there are MDV type-specific epitopes that are not shared by HVT or type 2 MDV have led us to the conclusion that improved vaccines might be constructed which are antigenically more related to MDV than existing vaccines. [Reviewed by Ross and Biggs in Goldman J. M. and Epstein M. A. (eds) Leukaemia and Lymphoma Research, Vaccine Intervention against Virus-Induced Tumour, p 13-31, Macmillan, 1986].
Infectious laryngotracheitis is also a worldwide problem. Sporadic outbreaks occur in which the severity of clinical symptoms varies considerably. Virus can persist in birds that have recovered and may be shed at intermittent intervals after recovery. An attenuated field strain is currently used as a vaccine. However, it has retained some degree of pathogenicity. Mortality due to the vaccine may reach 10% in young chicks.
A number of herpesvirus antigens have been shown to confer protective immunity when expressed in a recombinant vaccinia virus. These include the gB gene of HSV [Cantin E. M. et al (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 5908-5912], gD of HSV [Paoletti, E. et al (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 193-197] and gp50 of pseudorabies virus (PRV), a homologue of HSV gD [Marchioli, C. C. et al (1987) J. Virol. 61, 3977-3981]. Because of the absolute requirement of gB for virus penetration and infectivity and because it is conserved among herpes-viruses, gB and its homologues are important immunogens. Moreover, the presence of gB at the surface of infected cells has been shown to be an important target for humoral and cell-mediated immune responses [Blacklaws, B. A. et al J. gen. Virol. 68, 1103-1114 (1987); McLaughlin-Taylor, E. et al (1988) J. gen. Virol. 69, 1731-1734]. The recently described glycoprotein gH of HSV is also essential for infectivity and may also be an important immunogen [Desai, P. J. et al (1988) J. gen. Virol. 69, 1147-1156]. It has also been shown that gIII of pseudorabies virus (PRV), a homologue of gC, is a major target for neutralizing antibody and for cytotoxic T cells although it is a non-essential protein. Also of interest is the unexpected participation of immediate early proteins in T cell mediated cytotoxic reactions in cells infected with cytomegalovirus (CMV) [Kozinowski U. H. et al (1987) J. Virol. 61, 2054-2058]. Similar antigens could play an important role in the rejection of latently infected and transformed lymphocytes in Marek's disease since immediate early RNA transcripts have been detected in lymphoblastoid cell lines established from Marek's disease tumours.
Although many recombinant vaccines have been constructed using the poxvirus vaccinia as a vector, there are also reports of the use of herpesviruses as vectors for the expression of foreign genes. Thus hepatitis antigen has been expressed in HSV .epsilon.Shih, M. F. et al (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 5867-5870] and human tissue plasminogen activator has been expressed in PRV [Thomsen, D. R. et al (1987) Gene 57, 261-265. In both cases, foreign genes were inserted in cloned fragments of non-essential herpes genes which were then introduced into the virus vector by homologous recombination. The hepatitis virus gene was fused to a herpesvirus promoter and the recombinant DNA was inserted within the TK gene of HSV. Homologous recombination following co-transfection of the recombinant DNA and wild-type HSV DNA resulted in TK- virus clones that expressed the hepatitis antigen.
In the case of PRV, the gX gene mapping in U.sub.s was used as the site for insertion of the foreign gene. The strategy used involved insertion of the TK gene of HSV in the gX gene of a PRV mutant that had a defect in its TK gene resulting in a TK positive virus. The human tissue plasminogen activator gene was then inserted within a cloned fragement of HSV TK and the recombinant was introduced into the PRV mutant by homologous recombination. TK- virus was selected which expressed the human gene (Thomsen et al as above). Similarly, VZV has been used as a vector [Lowe et al (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 3896-3900].
Several herpesvirus genes have also been shown to be associated with virulence and to be non-essential for growth in vitro. These include the TK genes of HSV [Jamieson, A. T. et al (1974) J. gen. Virol. 24, 465-480; Field, H. and Wildy, P., (1987) J. Hygiene (Cambridge) 81, 267-277] and of PRV. Indeed it has long been known that PRV is readily attenuated by deletion of TK activity [Tatarov, G. (1968) Zentralbl. Vet. Med 15B, 848-853]. Furthermore, attenuation of the Bartha strain of PRV has been attributed to a defect in gI, a non-essential structural glycoprotein mapping in U.sub.s [Mettenleiter, T. et al (1987) J. Virol. 61, 4030-4032].
Genes of HSV mapping in the internal repeat region (TRS) flanking the long unique sequence have also been associated with pathogenicity [Rosen, A. et al (1986) Virus Research 5, 157-175; Thompson, R. L. et al (1983) Virology 131, 180-192]. Several additional genes of HSV have been shown to be non-essential for growth in vitro although it is not known whether they are associated with virulence. These include UL24 (Sanders, P. G., (1982), J. gen. Virol. 63, 277-295, large subunit of ribonucleotide reductase (Goldstein D. J. and Weller, S. K. (1988) J. Virol. 62, 196-205), gC (Draper K. G. et al (1984) J. Virol. 51, 578-585), dUTPase (Fisher, F. B. & Preston, V. G. (1986) Virology 148, 190-197), and U.sub.L 55 and U.sub.L 56 (MacLean, A. R. & Brown, S. M. (1987) J. gen. Virol. 68, 1339-1350).
Moreover there is evidence that several genes of HSV mapping in Us are also non-essential for growth in vitro [Weber, P. C. et al (1987) Science 236, 576-579].